Oh well… same problem other way round, now on the older i5-4570 machine I just upgraded to a 4K UHD screen with a MSI GeForce GT 1030 2G LP OC, LMDE2 Mate, proprietary drivers via sgfxi.
On this machine vlc runs fine, ≈6 % CPU at FHD, ≈70% CPU at 4K UHD. But mpv / mplayer / totem do not run at all…

It would help to know more about your system setup. If you run "inxi -Fxzd" from the console terminal prompt, highlight the results, copy and paste them back here, that should provide enough information.

From what you did post, you do not have the correct Nvidia video drivers installed which should be "nvidia-384" for a GT1030 video card.

To install this using the PPA method, open a console terminal, type in, or copy & paste, each line below one by one: Click "Select All" above command, right click the highlighted command, select Copy (or Ctrl+Insert), click in the console terminal window, and right click paste (or Shift+Insert), repeat for each command.

Huh? this is LMDE viz Debian Ed., not Ubuntu, hence rather no “PPA method”?
I had installed nvidia 387.34 via sgfxi, which is in my experience very competent in fixing Nvidia driver problems. But here seems to be something wrong, I just now noticed inxi's »GLX Renderer: N/A GLX Version: N/A«. I reran sgfxi and it seemed to succeed, but still no GLX and mpv not working.